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Forking

theft can optionally fork and run the property function in a child process, so that it can shrink failures that cause crashes or infinite loops.

Forking is configured via the .fork struct nested inside of struct theft_run_config:

    .fork = {
        .enable = true,      /* default: disabled */
        .timeout = TIMEOUT_IN_MSEC,  /* default: 0 (no timeout) */
        .signal = SIGTERM,   /* default: SIGTERM */
    },

Note that changes to memory in the child process will not be visible to the parent process, due to copy-on-write.

Performance

The overhead of shrinking a repeatedly crashing failure can vary significantly between operating systems.

In particular, the CrashReporter on macOS slows down shrinking by several orders of magnitude, as it logs information for every single crashing process. Disabling the CrashReporter or running theft on a non-macOS virtual machine can improve performance.

Timeouts

If forking is enabled, the .timeout field can be used to configure a timeout for each property trial (in milliseconds). If .timeout is nonzero, theft will kill(2) the child process if the test does not complete within the timeout. The kill signal defaults to SIGTERM, but can me configured via the .signal field.

After sending the signal, theft will wait for the child process to exit. If the test needs custom cleanup code, then send a signal such as SIGUSR1 to the test instead. Early in the property function, register a signal handler that will clean up and exit.

If the child process handles the signal and then returns THEFT_TRIAL_PASS or calls exit(EXIT_SUCCESS), then the trial will still be considered a PASS, otherwise the trial will be considered a FAIL. Signals that kill the process (such as SIGTERM or SIGKILL) will always be considered a FAIL.